Ranieri signed Duff for Chelsea in 2003, a move sanctioned by Roman Abramovich who had just bought the club, and the Italian has led Leicester to the top of the Premier League table this season against all the odds.

"This time last season they were bottom of the table, so I have not seen anything like that in any league, let alone the Premier League, so he has done an amazing job," the 36-year-old told Reuters.

Since retiring in December, Duff is trying to keep fit and is taking his coaching badges. He is set to start studying for his 'A' license next week and hopes to learn from Ranieri.

"He just gets people playing and working hard for him and that is half the battle when you are a football manager," Duff said.

"You can see the camaraderie within the squad and he has obviously created that and he is just a guy that you want to work for.

"He's a great guy and he gets you playing for him as you can see what is happening at Leicester City," said Duff who is playing in the Legends Cup in Moscow.

"No one expected it and I think I lot of people still expect him to not win the title, so it will be interesting to see how they finish."

Leicester are five points clear at the top going into Sunday's game against third-placed Arsenal, well placed to win the English top-flight title for the first time after narrowly avoiding relegation last season.